Method of producing glass substrates with improved long-term rigidity at elevated temperatures

ABSTRACT

Described is a process for the manufacture of glass substrates with improved long-term stability at raised temperatures, in which process a glass substrate is provided with a coating, this coating being prepared from a composition, which was obtained through hydrolysis and condensation of a compound that was dissolved in a solvent and belonged to at least one element from the group Si, Al, Ti and Zr and/or a suitable precondensate, optionally in combination with compounds, which are soluble in the reaction medium, of at least one element from the group of alkali metals, alkali earth metals, and boron. After the coating composition is deposited, the coating obtained thus is heat treated. The process is characterized in that the coating is not completely compacted.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a process for the manufacture of glasssubstrates with improved long term stability at raised temperatures, inparticular a process, in which a glass substrate is provided with acoating, which exhibits a certain residual porosity and, therefore,experiences a volume contraction subject to the influence of hightemperatures.

It is well-known that glass can be provided with a coating comprisingSi-, Ti-, Zr-, Al-based materials, etc. in order to improve thedifferent properties of the base glass. In principle, a covering layeris deposited on the base glass via a wet-chemical process (e.g. sol-gelprocess); and this layer is compacted into a film without pores by meansof heating. It is also well-known that such coatings increase; e.g., thestrength of the glass, especially when their coefficient of expansion isless than that of the base glass. In so doing, one can distinguishbetween two mechanisms. The one consists of reducing the number ofdefects on the glass surface and the other consists of formingcompressive strains in the region near the surface when said layer iscooled following condensation owing to the layer's lower thermalcoefficient of expansion. Since such compressive strains generallyincrease the strength of glass, it also affects the thermal stability,since the strains that occur at raised temperatures do not result in abreak until there are higher stresses. One has had similar experienceswith thermally or chemically hardened glasses; they, too, arecharacterized by a higher stability to thermal shock. Therefore, suchglasses are used in fields, where they must resist a thermal stress,e.g. during fire proofing.

Whereas there already exist methods for improving the thermal shockproperty of glasses, no one has succeeded to date in providing glasseswhose long-term stability at raised temperatures is satisfactory, thus,developed, for example, a glass pane that does not start to flow awaywhen it is subjected to a raised temperature over a prolonged period oftime. Especially glasses that are easy and inexpensive to manufacture(and are preferred, therefore, for most applications), such as sodiumlime glasses, usually exhibit, on account of their chemical composition,a low glass transition temperature or a low viscosity at temperatures,to which this glass is subjected within the framework of a specificapplication. Even at comparatively mild temperatures, such glasses canpresent a problem with respect to softening and flowing away. Examplesof glasses that can be subjected to high temperatures (optionally incombination with corrosive conditions) are furnace windows, fire proofglazing, optical glasses and chemical containers.

Thus, the present invention is based on the problem of providing aprocess, with which, starting from the most inexpensive base glass onecan obtain in an inexpensive manner glass substrates with improved flowbehavior, i.e. improved long-term stability at raised temperatures.

This problem is solved according to the invention by producing, not asin the case of the conventional process for improving the stability tothermal shock a compressive strain, but rather a tensile stress, withthe aid of a coating on the surface of the glass substrate, and inparticular during the heating up or heating period with the hightemperature stress. This process takes place when layers, which have atendency toward volume reduction subject to the effect of hightemperatures owing to their thermomechanical or thermochemicalproperties and thus produce tensile stresses on the surface of the glasssubstrate, are deposited on the glass substrate. Such layers can beproduced if a coating is produced by the conventional methods on theglass substrate, but said coating is not completely compacted. In otherwords, the coating still exhibits a certain residual porosity after itsmanufacture.

DETAILED DESCRIPTION

Correspondingly the invention provides a process for manufacturing glasssubstrates with improved long-term stability at raised temperatures, inwhich process

(a) a coating composition--which was obtained through hydrolysis andcondensation of at least one liquid compound and/or compound that isdissolved in a solvent and belongs to at least one element from thegroup Si, Al, Ti and Zr and/or a suitable precondensate, optionally incombination with compounds, which are soluble in the reaction medium, ofat least one element from the group of alkali metals (e.g. Na, K, Li),alkali earth metals (e.g. Ca, Mg, Ba) and boron and optionally in thepresence of a condensation catalyst--is deposited on the glasssubstrate; and

b) the coating obtained thus is heat treated.

This process is characterized in that the coating is not completelycompacted; i.e., it still exhibits a certain residual porosity (e.g. inthe range of 1 to 10% by volume) at the end of the heat treatment. Avery high percentage of this residual porosity comprises in thepreferred embodiment in essence exclusively micropores with a pore sizein the lower nm range (≦20 nm).

Preferably the coating composition is produced according to the sol/gelprocess.

Especially suitable for the above purpose are microporous layers. Suchlayers are, e.g., those with a relatively high SiO₂ content (e.g. atleast 80 and in particular at least 90 mole % Si, based on the centralatoms that are present altogether). Such layers have a high interfaceenergy and can be compacted upon heating. However, a microporosity canalso be produced by using, e.g. an alkyltrialkoxysilane compound as astarting compound for the coating composition. The alkyl group thatcannot be hydrolyzed is then not present in the finished coating, butcan be removed, e.g. thermally, thus leaving a "gap" (pore) behind. Inaddition, the use of such a compound (e.g. methyltriethoxysilane) makesthe coating less brittle (increase in the plasticity) and is alsopreferred for this reason. Preferably more than 50, in particular morethan 70 mole % of such starting compound are used. Even the addition of,e.g. silica sol, which process is described in detail below, canincrease the porosity of the coating, even though in this case it doesnot involve (the preferred) microporosity.

Suitable base glasses for the glass substrates, which are to be coatedaccording to the above process, are especially silicate glasses (e.g.window glass and bottle glass), lead silicate glasses, phosphateglasses, borate glasses, and rare earth glasses (La, Ce, Y and the like)with or without alkali and/or alkali earth components (e.g. Na, K, Ca,Ba). However, other glasses can also be coated according to the processof the invention, e.g. halogenide glasses (e.g. fluoride glasses) andchalcogenide glasses (e.g. sulfate glasses).

The glass substrate can be present in any arbitrary physical form, e.g.as boards, rods, tubes or fibers or as hollow bodies. In a specialembodiment glass or ceramic fiber cloth is impregnated with the coatingcomposition according to the invention, resulting in materials that areespecially suitable for purposes of fire proofing.

According to the invention, SiO₂, Al₂ O₃, ZrO₂, TiO₂, SiO₂ /TiO₂, SiO₂/ZrO₂, B₂ O₃ /SiO₂, Na₂ O/B₂ O₃ /SiO₂ and Na₂ O/Al₂ O₃ /B₂ O₃ /SiO₂compositions are preferred coating compositions. Especially preferredare SiO₂, SiO₂ /ZrO₂ and SiO₂ /TiO₂ coating compositions.

Suitable starting compounds for the coating composition are liquidcompounds and/or compounds that dissolve in the reaction medium(solvent) and that belong to said elements, e.g. hydroxides, alkoxides,halogenides (fluorides, chlorides, bromides, iodides), acids, salts withorganic or inorganic acids or bases and complex compounds or additioncompounds.

Concrete examples for the above compounds are hydroxides, like NaOH,KOH, Mg(OH)₂, Ca(OH)₂ and Al(OH)₃ ; hydrolyzable alkoxides, like NaOR,KOR, Mg(OR)₂, Ca(OR)₂, Ai(OR)₃, B(OR)₃, Si(OR)₄, RSi(OR)₃, R₂ Si(OR)₂,Ti(OR)₄ and Zr(OR)₄, where R is a straight chained or branched,substituted or non-substituted alkyl group having 1 to 6 (preferably 1to 4) carbon atoms (e.g. methyl, ethyl, propyl or butyl); halogenides,like NaCl, KBr, CaCl₂, SiCl₄, R'SiCl₃, R'₂ SiCl₂, TiCl₄ and ZrCl₄(R'=alkoxy group derived from the above alkyl groups); acids, like boricacid; salts, like borates, nitrates, phosphates, carboxylates (e.g.formiates, acetates, citrates, tartrates, basic acetates and bonzoates)and phenolates; and complex compounds, like acetylacetonates andsalicylates and addition compounds such as Zr(NO₃)₄ dioxan, THF,pyridine and the like.

Preferably one of the starting compounds is a compound of the generalformula (I):

    Si(OR).sub.4                                               (I)

where R is defined as above.

In another preferred embodiment of the present invention the startingcompounds include not only those of the above formula (I) but also thoseof the general formula (II):

    R*Si(OR).sub.3                                             (II)

where R is defined as above and R, stands for R or aryl, in particularphenyl.

The starting compounds are dissolved preferably in water or an organicsolvent or a mixture comprising water and an organic solvent that can bemixed with water (preferably an alcohol) and subjected to hydrolyticpolycondensation. Organic solvents are with respect to the targetedimproved coating properties preferred, whereby especially preferred aresolvents that can be mixed at least partially with water. Examples ofthe suitable organic solvents are monovalent and polyvalent alcohols,like methanol, ethanol, n-propanol, isoprspanol, n-butanol,tert-butanol, ethylene glycol and glycerol; ethers, likedimethoxyethane, tetrahydrofuran, and ethylene glycol monomethyl ether;esters, like ethyl acetate and diethylene glycol acetate; ketones, likeacetone and methyl ethyl ketone; hydrocarbons, like benzene and toluene;halogenated hydrocarbons, like carbon tetrachloride; and dimethylformamide. Of the organic solvents, alcohols are preferred.

The hydrolysis and polycondensation can be conducted in one step in thepresence of at least the stoichiometric amount of water required forcomplete hydrolysis of the existing hydrolyzable groups. As analternative a precondensation in the presence of a small amount of wateras the stoichiometric amount required for complete hydrolysis or of theexisting hydrolyzable groups can be conducted; and then furthercondensation can be conducted with the addition of at least that amountof water that is needed to hydrolyze the remaining hydrolyzable groups.

The polycondensation may or may not be conducted in the presence of acondensation catalyst, e,g. a compound, which splits off protons orhydroxyl ions, or an amine. Specific examples are organic and inorganicacids, like hydrochloric acid, sulfuric acid, phosphoric acid, formicacid and acetic acid, as well as organic and inorganic bases, likeammonia, alkali and alkali earth metal hydroxides (e.g. NaOH, KOH orCa(OH)₂) and amides, which are soluble in the reaction medium, e.g. lowalkyl amines and alkanol amines. Especially preferred are volatile acidsand bases in particular hydrochloric acid, ammonia and triethylamine.The total catalyst concentration can range, e.g., up to 5 mole/liter.

The concentration of starting compound(s), based on the solvent, isadjusted in such a manner that a viscosity of the coating compositionthat is suitable for the subsequent coating operation is guaranteed.

The hydrolysis and partial precondensation is conducted usually at roomtemperature, but higher temperatures can also be applied, preferably upto the boiling point of the solvent that is used. The reactionconditions are adjusted in such a manner that the resulting coatingsolution has a viscosity of, e.g. 1 to 5 mPas, that is suitable fordepositing a thin layer on the base glass.

The resulting coating solution (coating composition) is then depositedby the conventional method on the glass substrate to be treated,preferably by dipping, drawing or spraying. Especially preferred aredipping methods.

The amount of coating is chosen in such a manner that the layerthickness of the coating obtained following heat treatment ranges from0.8 to 8 μm, preferably 1.5 to 3 μm, e.g. 2 μm.

Especially if layer thicknesses exceeding 2 μm are targeted, preferablya dispersion of, e.g., SiO₂, TiO₂, Al₂ O₃ and/or ZrO₂ or thecorresponding mixed oxides in water and/or another suitable solvent isadded to the coating solution, or is included in the production process.The particle size of the added dispersions depends on the intendedapplication, but ranges preferably from 4 to 20 nm.

Rheologically active additives can also be added in the customaryamounts to the coating solution. Examples of such additives arecellulose derivatives, like methyl cellulose and hydroxypropylcellulose, ethylene glycol, glycerol, polyethylene glycol, polyvinylacetate, polyvinyl alcohol and the like.

During the glass production, the coating can be applied directly on thestill hot base glass from the finishing process, e.g. by means ofaerosol coating. In this case, a separate drying and heat treatment stepis superfluous. On the other hand, e.g. during the coating operation ofa glass substrate, which is supposed to be coated subsequently, thecoating is dried at normal pressure or reduced pressure and atemperature around the boiling point of the solvent used, e.g. 10minutes at 110° C.

In the subsequent process step, the coating is heat treated at atemperature below the softening point of the base glass, preferablybelow 550° C., in such a manner that the coating is not completelycompacted. The treatment temperature is chosen in such a manner that theshape does not change. For flat glass (window glass) this means, e.g.,that the temperature should not exceed 500° C.

If the glass substrate produced in this manner is subjected at the siteof application, e.g., as furnace windows or fire proof glazing, to hightemperatures, the result is a volume contraction, as described above,during the heating up period owing to the existing residual porosity;and a tensile stress is produced in the coating, which is responsiblefor the targeted improvement in the flow behavior.

The following examples serve to further explain the invention, withoutrestricting it.

EXAMPLE 1

    ______________________________________                                        Preparation of a SiO.sub.2 sol                                                Required chemicals:                                                           ______________________________________                                        20         ml         methyltriethoxysilane                                   6          ml         tetraethoxysilane                                       6          ml         water                                                   6          ml         propanol                                                0.02       ml         HCl (conc.)                                             ______________________________________                                    

The alkoxysilanes are mixed while stirring; and 3 ml of propanol areadded. Water, the remaining 3 ml of propanol and the concentratedhydrochloric acid are mixed and left to merge slowly while stirring.After 20 minutes the sol is cooled to room temperature and is filteredthrough a 0.2 μm injection filter.

Window glass is coated with the aid of an immersion coating apparatus(speed of draw 1-8 mm/s). The freshly drawn layers are dried at 60° C.for 15 minutes and subsequently compacted (incompletely) in a furnaceaccording to the following temperature program:

room temperature to 400° C. with 1K/min

400° C. to 500° C. with 0.3K/min.

These temperature conditions were not sufficient to compact completelythe coating. Rather many small micropores which were not visible to thenaked eye remained in the coating. The thickness of the finished coatingwas approx. 1.5 μm.

EXAMPLE 2

    ______________________________________                                        Preparation of a sol containing TiO.sub.2                                     Required chemicals:                                                           ______________________________________                                        16.8      ml         methyltriethoxysilane                                    2.3       ml         tetraethoxysilane                                        8         ml         water                                                    4         ml         titanium(IV)isopropylate                                 21        ml         ethanol                                                  0.04      ml         HCl (conc.)                                              ______________________________________                                    

The alkoxysilanes are mixed; and 2 ml of ethanol, 0.04 ml of HCl, and 1ml of water are added. After 5 minutes of vigorous stirring, 4 ml oftitanium(IV)isopropylate are added and the mixture is stirred for a fewmore minutes. Then, 7 ml of water and 19 ml of ethanol are added; andthe resulting mixture is filtered after 20 minutes through an 0.8 μminjection filter.

Window glass is coated with the aid of an immersion coating apparatus(speed of draw 1-6 mm/s). The freshly drawn layers are dried at 60° C.for 15 minutes and subsequently compacted in a furnace according to thefollowing temperature program:

room temperature to 400° C. with 1K/min

400° C. to 500° C. with 0.3K/min.

The thickness of the finished coating ranged from 1-1.5 μm.

EXAMPLE 3

    ______________________________________                                        Preparation of a coating sol containing silica sol                            Required chemicals:                                                           ______________________________________                                        20      ml       methyltriethoxysilane                                        6       ml       tetraethoxysilane                                            15      g        of Bayer silica sol type 300, 30% by wt.                                      (concentrated to 45% by wt.)                                 0.3     ml       HCl (conc.)                                                  5       ml       propanol                                                     ______________________________________                                    

The silanes are introduced and the silica sol is added with vigorousstirring. After an emulsion has formed (approx. 20 sec.), thehydrochloric acid is added in order to start the hydrolysis. The mixtureremains cloudy for 20-60 seconds and then suddenly becomes firstviscous, then liquid and then clear. During this reaction, the sol heatsup to approx. 40° C. After cooling to room temperature, the mixture isfiltered through an 0.8 μm filter with 5 μm prefilter. The sol producedthus can be adjusted with ethanol to a desired viscosity and isavailable for coating for at least 6 hours.

Window glass is coated with the aid of an immersion coating apparatuswith speeds of draw ranging from 3-10 mm/s. The freshly drawn layers aredried at 60° C. for 15 minutes and subsequently compacted in a furnaceaccording to the following temperature program:

room temperature to 400° C. with 1K/min

400° C. to 500° C. with 0.3K/min.

The thickness of the finished coating ranged from 2-6.5 μm.

EXAMPLE 4

    ______________________________________                                        Preparation of a coating sol containing zirconium oxide                       Required chemicals:                                                           ______________________________________                                        19.2    ml      methyltriethoxysilane                                         2.6     ml      tetraethoxysilane                                             5.1     ml      of H.sub.2 O                                                  2.7     ml      of zirconium(IV)butylate (80% in n-butanol)                   10.0    ml      propanol                                                      0.05    ml      HCl (conc.)                                                   ______________________________________                                    

The silanes are introduced and prehydrolyzed with hydrochloric acid and1 ml of water for 5 minutes. To homogenize the solution, another 3 ml ofpropanol are added. After 5 minutes have elapsed, a mixture comprising 3ml of propanol and 2.7 ml of zirconium(IV) burylate is added to theprehydrolysate while stirring vigorously. After one minute, theremaining water (4.1 ml), mixed with 4 ml of propanol, can be added.After the solution has been cooled to room temperature and filteredthrough an 0.8 μm filter, it can be used for coating for at least 8hours.

Window glass is coated with the aid of an immersion coating apparatuswith speeds of draw ranging from 1-6 mm/s. The freshly drawn layers aredried at 60° C. for 15 minutes and subsequently compacted in a furnaceaccording to the following temperature program:

room temperature to 400° C. with 1K/min

400° C. to 500° C. with 0.3K/min.

The thickness of the finished coating ranged from 1-1.5 μm.

EXAMPLE 5 Preparation of an Impregnated Glass Fiber Cloth

200 ml of methyltriethoxysilane, 60 ml of tetraethoxysilane and 71 ml of30% silica sol (Bayer silica sol type 300) are stirred vigorously; and1.8 ml of concentrated hydrochloric acid is added. After 10 minutes thesolution is diluted with 50 ml of ethanol.

Coating Process 1

The sol is filled into a polyethylene immersion tank. A glass fiberfabric (30×60 cm) from commercially available E glass having a weight of400 m² /g is dipped into the solution and pulled out after 1 minute ofdwell time at a speed of draw of 6 mm/s. Subsequently the fabric is ovendried for 20 minutes at 100° C.

Coating Process 2

The sol is further diluted with 100 ml of ethanol and sprayed with acompressed air atomizer on both sides of the free hanging glass fibercloth until the fabric is saturated. Then the fabric is dried with a hotair blower at approx. 100° C. of hot air.

After this treatment the glass fiber cloth resists the effect of 2butane gas burners (Tmax: 1,750° C.). To quantify the reinforcing effecta 10×5 cm piece of cloth is examined in a measuring apparatus, whichrecords the elongation of the cloth under temperature stress. In sodoing, the uncoated fabric exhibited a softening point of approx. 840°C., whereas the coated test sample did not tear until approx. 1,050° C.

We claim:
 1. A process for the manufacture of a coated glass substrate,comprising the steps of:(i) preparing a coating composition byhydrolyzing and condensing:(a) at least one compound of formula (I):

    Si(OR).sub.4                                               (I)

wherein R stands for a C₁₋₆ alkyl group; and(b) at least one compound offormula (II):

    R*Si(OR).sub.3                                             (II)

wherein R is as defined above and R* represents R or a C₆₋₁₄ aryl group;and, optionally,(c) at least one other compound of an element selectedfrom the group consisting of Si, Al, Ti and Zr; and/or (d) suitableprecondensates of said compounds (a) to (c); or a combination of saidcompounds (a) to (c) with said precondensate;said compounds (a) to (c)and said precondensates (d) being either liquids or being dissolved in asolvent or both; optionally in combination with at least one compound ofan element selected from the group consisting of alkali metals, alkalineearth metals and boron, said at least one compound being soluble in thereaction medium; and optionally in the presence of a condensationcatalyst; (ii) applying said coating composition of step (i) onto aglass substrate; and (iii) heat treating the coated glass substrate ofstep (ii) so as to not completely compact the coating, thereby preparinga coated glass substrate of improved long-term stability at elevatedtemperatures.
 2. The process of claim 1, wherein the coating compositionis produced by the sol/gel process.
 3. The process of claim 1, whereinsaid glass substrate is a silicate glass, lead silicate glass, phosphateglass, borate glass or a rare earth glass with or without alkali,alkaline earth or mixed alkali-alkaline earth components.
 4. The processof claim 1, wherein the coating composition further is based upon astarting compound containing aluminum, titanium or zirconium.
 5. Theprocess as claimed in claim 1, wherein the solvent is water or awater-alcohol mixture.
 6. The process of claim 1, wherein the hydrolysisand precondensation are conducted in the presence of an acidic or basiccondensation catalyst.
 7. The process as claimed in claim 1, wherein adispersion of SiO₂, TiO₂, Al₂ O₃, ZrO₂ or a mixed combination of oxidesin water, an organic solvent or a combination thereof is added to thecoating composition before, during, after or a combination thereof thepreparation of the same.
 8. The process as claimed in claim 1, whereinthe coating composition is applied by dipping, drawing or spraying. 9.The process as claimed in claim 1, wherein the coating is heat treatedat a temperature below the softening point of the base glass.
 10. Theprocess as claimed in claim 9, wherein said temperature of heattreatment is below 550° C.
 11. The process as claimed in claim 1,wherein the heat treated coating exhibits a thickness ranging from 0.8to 8 μm.
 12. The process as claimed in claim 11, wherein said thicknessranges from 1.5 to 3 μm.
 13. The process as claimed in claim 1, whereinR is a C₁₋₄ alkyl group, and R* is phenyl.
 14. A glass substrate whichexhibits improved long-term stability at elevated temperatures preparedby the process of claim
 1. 15. A furnace window, fire proof glazing,fire proof fire mat, optical glass or chemical container formed of theglass substrate of claim 14.